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<a name="Order-of-Allocation-of-Registers"></a>
<h4 class="subsection">18.7.2 Order of Allocation of Registers</h4>
<a name="index-order-of-register-allocation"></a>
<a name="index-register-allocation-order"></a>

<p>Registers are allocated in order.
</p>
<dl>
<dt><a name="index-REG_005fALLOC_005fORDER"></a>Macro: <strong>REG_ALLOC_ORDER</strong></dt>
<dd><p>If defined, an initializer for a vector of integers, containing the
numbers of hard registers in the order in which GCC should prefer
to use them (from most preferred to least).
</p>
<p>If this macro is not defined, registers are used lowest numbered first
(all else being equal).
</p>
<p>One use of this macro is on machines where the highest numbered
registers must always be saved and the save-multiple-registers
instruction supports only sequences of consecutive registers.  On such
machines, define <code>REG_ALLOC_ORDER</code> to be an initializer that lists
the highest numbered allocable register first.
</p></dd></dl>

<dl>
<dt><a name="index-ADJUST_005fREG_005fALLOC_005fORDER"></a>Macro: <strong>ADJUST_REG_ALLOC_ORDER</strong></dt>
<dd><p>A C statement (sans semicolon) to choose the order in which to allocate
hard registers for pseudo-registers local to a basic block.
</p>
<p>Store the desired register order in the array <code>reg_alloc_order</code>.
Element 0 should be the register to allocate first; element 1, the next
register; and so on.
</p>
<p>The macro body should not assume anything about the contents of
<code>reg_alloc_order</code> before execution of the macro.
</p>
<p>On most machines, it is not necessary to define this macro.
</p></dd></dl>

<dl>
<dt><a name="index-HONOR_005fREG_005fALLOC_005fORDER"></a>Macro: <strong>HONOR_REG_ALLOC_ORDER</strong></dt>
<dd><p>Normally, IRA tries to estimate the costs for saving a register in the
prologue and restoring it in the epilogue.  This discourages it from
using call-saved registers.  If a machine wants to ensure that IRA
allocates registers in the order given by REG_ALLOC_ORDER even if some
call-saved registers appear earlier than call-used ones, then define this
macro as a C expression to nonzero. Default is 0.
</p></dd></dl>

<dl>
<dt><a name="index-IRA_005fHARD_005fREGNO_005fADD_005fCOST_005fMULTIPLIER"></a>Macro: <strong>IRA_HARD_REGNO_ADD_COST_MULTIPLIER</strong> <em>(<var>regno</var>)</em></dt>
<dd><p>In some case register allocation order is not enough for the
Integrated Register Allocator (<acronym>IRA</acronym>) to generate a good code.
If this macro is defined, it should return a floating point value
based on <var>regno</var>.  The cost of using <var>regno</var> for a pseudo will
be increased by approximately the pseudo&rsquo;s usage frequency times the
value returned by this macro.  Not defining this macro is equivalent
to having it always return <code>0.0</code>.
</p>
<p>On most machines, it is not necessary to define this macro.
</p></dd></dl>

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